1. Technical Field
The present disclosure relates to apparatus and methods for inspecting and analyzing semiconductor wafers and other substrates using scatterometry and related techniques.
2. Description of the Background Art
Scatterometry refers to an optical technique that analyzes diffracted light to deduce structural details of a diffracting sample. The diffracting sample is generally a periodic structure, that is, a “grating.” Scatterometry may be used to measure or analyze two-dimensional structures (line gratings), as well as three-dimensional structures (such as periodic patterns of mesas or vias on a substrate).
FIG. 1A is a schematic view of a spectroscopic scatterometer system 10. As shown in FIG. 1A, system 10 may be used to measure reflected or transmitted intensities or changes in polarization states of the diffracted light. As shown in FIG. 1A, a semiconductor wafer 11 may comprise a silicon substrate 12, and a structure 16 thereon that may include a photoresist pattern on and/or over film stack(s), where the film(s) are at least partially light-transmissive and has a certain film thickness and refractive index (n and k, the real and imaginary components of the index).
An XYZ stage 14 is used for moving the wafer in the horizontal XY directions. Stage 14 may also be used to adjust the z height of the wafer 11. A broadband radiation source such as white light source 22 supplies light through a fiber optic cable 24 which randomizes the polarization and creates a uniform light source for illuminating the wafer. Preferably, source 22 supplies electromagnetic radiation having wavelengths in the range of at least 180 to 800 nm. Upon emerging from fiber 24, the radiation passes through an optical illuminator that may include an aperture and a focusing lens or mirror (not shown). The aperture causes the emerging light beam to image a small area of structure 16. The light emerging from illuminator 26 is polarized by a polarizer 28 to produce a polarized sampling beam 30 illuminating the structure 16.
The radiation originating from sampling beam 30 that is reflected by structure 16, passed through an analyzer 32 and to a spectroscopic ellipsometry (SE) spectrometer 34 to detect different spectral components of the reflected radiation, such as those in the spectrum of the radiation source 22, to obtain a signature of the structure. In one mode (spectrophotometry mode) of operation, the reflected intensities are then used in a manner described below to find the value(s) of one or more parameters of structure 16. The system 10 can also be modified by placing the spectrometer 34 on the side of structure 16 opposite to illumination beam 30 to measure the intensities of radiation transmitted through structure 16 instead for the same purpose. These reflected or transmitted intensity components are supplied to computer 40. Alternatively, the light reflected by the structure 16 is collected by lens 54, passes through the beam splitter 52 to a spectrometer 60. The spectral components at different wavelengths measured are detected and signals representing such components are supplied to computer 40. The light reflected by structure 16 may be supplied by source 22 through illuminator 26 as described above or through other optical components in another arrangement. Thus, in such arrangement, lens 23 collects and directs radiation from source 22 to a beam splitter 52, which reflects part of the incoming beam towards the focus lens 54 which focuses the radiation to structure 16. The light reflected by the structure 16 is collected by lens 54, passes through the beam splitter 52 to a spectrometer 60.
When the system 10 is operated in another mode (spectroscopic ellipsometry mode) used to measure the changes in polarization state caused by the diffraction by the structure, either the polarizer 28 or the analyzer 30 is rotated (to cause relative rotational motion between the polarizer and the analyzer) when spectrometer 34 is detecting the diffracted radiation from structure 16 at a plurality of wavelengths, such as those in the spectrum of the radiation source 22, where the rotation is controlled by computer 40 in a manner known to those skilled in the art. The diffracted intensities at different wavelengths detected are supplied to computer 40, which derives the changes in polarization state data at different wavelengths from the intensities in a manner known to those in the art.
FIG. 1B is a cross-sectional view of an example structure 16 on substrate 12, which structure comprises a diffracting structure 16b situated between the film stack 16a above the structure and the film stack 16c underneath the structure, and an incident electromagnetic beam 30 to illustrate operation of the spectroscopic scatterometer system 10. Thus, the incident beam 30 of the electromagnetic radiation first encounters the interface between the air and the film stack 16a and interfaces that may be present within the stack. Next, the portion of the radiation from beam 30 that penetrates the film stack 16a is diffracted by the grating structure 16b. At least some of the radiation from beam 30 will reach the film stack 16c underneath the grating and be reflected by or transmitted through interfaces associated with stack 16c. The total light reflectance is affected both by the grating and by the film stacks above and/or below the grating. Multi-layer interference, caused by multiple reflections between the films and the grating, creates a complicated pattern in a reflectance spectrum, which can be used for measuring parameters of the structure. A part of radiation from beam 30 that is not reflected or diffracted as described above will be transmitted into the substrate 12. As shown in FIG. 1B, the grating 16b has a height of H, a critical dimension CD and a side wall angle (SWA) as indicated.